I can't tell you how to fix it, but I can give you a couple of
work-arounds. I have been having this problem for a couple of weeks.

You can drop back to the Update Manager window and click the "Check" button. It
will fail with a signature validation error, but if you click the "Check" button
again, it will run without errors. Alternatively, you can open a terminal
session and enter "sudo apt-get update". When you run the command, it will fail
with a signature validation error, but if you hit the up arrow and run the
command again, it will run without errors.

My guess is that dropbox has some issues.